“Three functions in one”: multifunctional rare-earth cyamelurates with magnetism, luminescence, and giant optical nonlinearity

A highly-integrated optical device requires multifunctional crystals, which respond strongly to external stimuli, such as photoluminescence, nonlinear polarization, and magnetoelectric coupling. However, it is usually difficult for these functions to coexist in a single-phase crystal, and can even be incompatible. Here, we propose a 'unit assembly' strategy in rare-earth cyamelurates, which combines active rare-earth ions (Gd³⁺, Y³⁺, Lu³⁺) and π-conjugated [H _<i>x</i> C₆N₇O₃]^{(3-<i>x</i>)-} units into one crystal with an aligned arrangement. For the first time, we synthesized three new rare-earth cyamelurate crystals RE(H_1.5C₆N₇O₃)₂·10H₂O (RE = Y, Gd, Lu) by a facile solution method. Benefitting from the strong polarizability and delocalized electrons of the π-conjugated units, all three crystals display giant second-order optical nonlinearity (12-15.3 × KDP), broadband photoluminescence (300-500 nm), and strong anisotropic birefringence (Δ<i>n</i> > 0.22) at 1064 nm. Moreover, the Gd-analogue exhibits paramagnetic behaviour in a wide temperature range. These results highlight rare-earth cyamelurates as promising multifunctional optical crystals with three concurrent functional responses, indicating potential applications in next-generation photonics and optoelectronics.